Increasing use is being made of wavelength multiplexing/demultiplexing techniques for the purpose of more effective utilization of the transmission capacity of optical conductors. In this case, several mutually independent optical signals of different wavelengths are transmitted over a common optical conductor. Multiplexing/demultiplexing techniques are then used to combine and separate these signals at the receiver and/or transmitter. One possibility for separating and/or combining optical signals of different wavelengths is to use narrowband optical bandpass filters.
Such filters mostly consist of dielectric layer systems which are mounted on suitable carrier substrates, generally glass substrates. Their transmission and/or return characteristics for a specific wavelength region are specifically set by selecting the thicknesses and refractive indices of the individual layers and their arrangement.
The reflectivity and/or transmissivity of optical bandpass filters of different center wavelengths is illustrated schematically and in an idealized fashion in FIG. 9 as a function of the wavelength of the light. In this case, xcex1 to xcexn stand for the center wavelengths of the filters F1 to Fn, xcex stands for the wavelength of the light, and R and T stand for the reflectance and transmittance. The transmittance is illustrated with unbroken lines, and the reflectance with dashed lines. It holds that R≈1xe2x88x92T. These filters have a particularly high transmission in a specific region about their center wavelength, but reflect light of wavelengths outside this region. They therefore act as wavelength-selective mirrors. The center wavelength is a function in this case of the angle of incidence of the light beams.
A multiplexing/demultiplexing arrangement having optical filters, which is known in the prior art, is illustrated in FIG. 10. The optical signal emerging from an optical conductor F1, as a rule a glass fiber, of wavelengths xcex1-xcexn is collimated by an imaging system L1, rendered parallel and coupled into a filter system. As a rule, the imaging system in this case comprises a lens, mostly a graded-index lens. The filter system comprises optical filters Fi1, Fi2, Fi3, which are fitted at a fixed spacing from one another onto the mutually opposite sides of a plane-parallel glass plate 8 and in a fashion offset relative to one another. The light coupled into the glass plate 8 by the imaging system L1 at a specific angle then runs to and fro inside the glass plate in a zigzag fashion between the mutually opposite filters Fi. Light of a specific wavelength is coupled out of the beam path at each filter Fi and coupled into an optical conductor F2, F3, F4 by the associated imaging system L2, L3, L4.
It is a disadvantage of such arrangements that a special filter of a specific center wavelength must be used respectively for each wavelength. Several different filters are required depending on the wavelength separation and bandwidth of the individual signals to be separated and/or combined, and in this case only very small fault tolerances are permissible in producing the filters. This leads, on the one hand, to an increased rejection of filters for specific wavelengths and thus to increased production costs and, on the other hand, requires high costs for storing individual special filters.
The object of the present invention is to make available a module for multiplexing and/or demultiplexing optical signals in which there is a reduction in the number of the filters of a different characteristic which are required for a multiplexing/demultiplexing arrangement, and in which it is possible to use filters with higher manufacturing tolerances.
It is provided in accordance therewith that at least one wavelength-selective filter can be set with reference to the angle of incidence of the light beams. The desired center wavelength of the respective filter can be set exactly in this case via the angle of incidence. This has two advantages. Firstly, individual special filters can be used as multiplexing elements for several wavelengths, the selected wavelength being set via the angle of incidence of the light beams. By using identical filters for different wavelength channels, it is possible to reduce the total number of different filters to be produced, and thus to lower the costs of production and storage.
Secondly, it is possible to use filters with higher tolerances with reference to the center wavelength, since the desired center wavelength can be set precisely by appropriately tilting the filter even in the case of high tolerance values. Consequently, the rejection in filter production can be substantially reduced, and costs can be saved correspondingly.
In a preferred refinement of the invention, the light is coupled into or out of the module by means of optical conductors, each optical conductor being assigned at least one imaging element which is arranged between the optical conductor and a filter. In this case, the optical axes of the optical conductors and the optical axes of the imaging elements assigned to the optical conductors are preferably arranged parallel to one another. Both the adjustment and the fixing of the individual components are facilitated in this way by a parallel arrangement.
In a first development of this refinement of the invention, in each case the imaging element is a lens which is transirradiated off-axis by the light of the assigned optical conductor. The beam deflection required for the functioning of the multiplexing/demultiplexing arrangement is achieved in this case by means of a parallel offset of the optical axis of the optical conductor and assigned imaging element. Such an arrangement is of particularly simple design and is therefore also simple in terms of production engineering and can be executed with a low outlay on adjustment. In this case, use is also made in some circumstances of more complicated, multistage lens systems, for example, to reduce imaging errors resulting from the off-axis transirradiation of the lens.
In a second development, in each case the imaging element is a lens which is transirradiated axially by the light of the assigned optical conductor. Imaging errors owing to off-axis transirradiation of the lens are thereby avoided. However, in order to retain the parallelism of the optical axes of the optical conductors there is then additional need for at least one optical element which can be tilted with reference to the angle of incidence of the light beam and deflects light reflected by the filter in the direction of the lens and the assigned optical conductor. The tiltable optical element is, in particular, a mirror or prism arranged in the beam path between the filter and lens. In this case, by comparison with the use of a prism, the use of a mirror has the advantage of avoiding an additional wavelength dependence on the basis of the dispersion of the glass. Mirrors, prisms and also the wavelength-selective filters should have a weak dependence on polarization.
The arrangements described for producing a beam deflection have the advantage that it is possible in a multichannel multiplexing/demultiplexing arrangement having cascades of optical conductors, imaging systems and tiltable filters to avoid the direct adaptation, which is very complicated in terms of production engineering, of the angular settings of the optical axes of the imaging systems and optical conductors to the tilted filter/filters.
Particularly compact designs are provided by arrangements in which either several filter cascades or filter and mirror/prism cascades are combined. The individual filters of a cascade can be filtered either individually or in common in this case. In the cascaded arrangements, the respective optical conductors are preferably arranged parallel to one another in accordance with the above-described beam deflecting arrangements, and can thereby easily be adjusted and fixed.
A first such advantageous arrangement is provided by two, mutually opposite filter cascades. The filters, which can be tilted about the beam axis, of the two cascades are mutually offset in this case, such that the beam path between the filter cascades describes a zigzag line. A specific wavelength is coupled out at each filter element in a wavelength-selective fashion and coupled into the appropriate optical conductor by the imaging system. The optical axis of the imaging system and the axis of the optical conductor are parallel to one another in this case. Beam deflection and the compensation of the tiltability of the filters are preferably achieved by an adjustable parallel offset between the optical axis of each imaging system and the respective optical conductor axis.
A second such advantageous arrangement is provided by the combination of a filter cascade with a mirror cascade. The elements, which can be tilted about the beam axis, of the two cascades are preferably arranged in this case offset relative to one another such that the beam path between the cascades describes a zigzag line. In this case, a specific wavelength is coupled out at each filter element and coupled into the appropriate optical conductor by the imaging system. The optical axis of the imaging system and the optical axis of the optical conductor preferably coincide in this arrangement, in order to avoid additional imaging errors. The required beam deflection and the compensation of the tiltability of the filter elements are achieved by the tiltable mirror elements.
A third advantageous arrangement is provided by the combination of a filter cascade and a mirror cascade, in the case of which the individual filters of a cascade are arranged one behind another. In this arrangement, an individual filter element of a cascade preferably reflects light of only one wavelength, all others being transmitted. The light reflected by a filter is deflected onto an element of the imaging system via a tiltable mirror of the mirror cascade. The optical axes of the imaging system and the optical conductor axis preferably coincide in this case. It is also preferred to provide that the light respectively deflected by a tiltable mirror is coupled out into the filter cascade essentially at right angles to the beam direction, such that the individual optical conductors are in turn arranged parallel to one another.
In a preferred refinement of the invention, all the optical channels are arranged on one side of the component or the filters. For this purpose, the optical channels of the first or the second optical imaging system are coupled into or out of the module via a deflecting prism, if appropriate. The arrangement of the optical channels on only one side of the module has advantages in terms of production engineering.
In an advantageous development of the invention, the filters and, if appropriate, mirrors or prisms are arranged on a flat platform, and this platform is inserted into a housing which has at least one light entry/exit port, the lenses and optical conductors of the first and/or second imaging system being permanently connected to the outside of the housing. In this case, the fastening and adjustment of the imaging systems and/or lens holders and optical fiber holders is preferably performed by means of laser welding technology at the fixed housing. This produces a fastening which is particularly stable in the long-term mechanically.
In particular, the fastening of the lenses/fiber elements is preferably performed by a means of a free active adjustment of the elements in a holder flange or a holder sleeve, and by subsequently welding these-flanges or sleeves to the housing.